If the main roads in Kent turn into tanker parks and this results in shortages of fuel then this could trigger a feedback loop. That loop would involve the food chain, not just for imports but for home manufactured goods too, which still needs transporting.
Yes, the Kent tanker parks have occurred before, but here we're talking all European routes into the UK, not just the channel.
The other thing to consider is the "hi-tech" UK/Eire border. Has anyone here got a friend who is working on the infrastructure for that? Can we find out how that project is going, please? No, I know nothing is set in stone yet, but even if that has been worked on for some years there are going to be glitches, and here on The Register, we are all well aware of how well-written infrastructure systems are implemented (do I need the sarcasm tags?). The fact is that Ireland has still got to import/export directly with the EU, currently it makes sense for that to happen through the UK, what happens after Brexshit? So we can add these elements to our feedback loop, which is additive to the feedback loop I started with. Except that this could have a detrimental effect on Ireland's ability to trade internationally, which is something that the EU will take a great interest in, being one of their member states is being affected.
Transport and basic living essentials have got to be ring-fenced, then everything should be stable. Goes without saying IMHO, but consider the situation that arose recently where there was a shortage of CO2. This seemed to catch the government unawares (correct me if I'm wrong), but they seemed to leave "market forces" to deal with it. Having been exposed to the stories about this I'm sure we can now agree that this situation should not be allowed to happen again - because it could impact adversely on the food chain.
My point? Oh yes, civil unrest. Well if there's queues for petrol and food then you're going to need to police those queues, judging by the behaviour of people buying on Black Friday two years ago.